The human interferon alphas are a family of proteins comprising at least 24 subspecies, Zoon K.C., Interferon 9, 1-12 (1987), Gresser I., ed. Academic Press, New York. They were originally described as agents capable of inducing an antiviral state in cells but are known as pleiotropic lymphokines affecting many functions of the immune system, Opdenakker, et al., Experimentia 45, 513-520 (1989). Apart from their in vitro biological activities the human Interferon alphas are currently used for several important indications, e.g. hairy cell leukemia, Kaposi Sarcoma, veneral warts, and are being investigated for several others Intron A (interferon alpha-2b) Clinical status (1989) Proceedings from a satellite symposium at the 5.sup.th European Conference on Clinical Oncology, London, U.K. September 1989. The demand for highly purified and crystalline forms of interferon alpha, especially the recombinant type alpha-2b is of foremost importance for structure elucidation as well as for formulation of vadous dosage forms.
Two forms of crystalline human interferon alpha-2 have been reported. Miller et al., Science, 215, 689-690 (1982); Kung et al., U.S. Pat. No. 4,672,108; Weissmann, The Cloning of interferon and other Mistakes, In: Interferon 1981, Ion Gresser, ed., Academic Press, New York, 101-134; Weissmann, Phil. Trans. R. Soc. Lond., B299, 7-28 (1982); and Nagabhusban, et al., Characterization of Genetically Engineered Alpha-2 Interferon, In: Interferon: Research Clinical Application and Regulatory Consideration, Zoon, et al., Elsevier, New York 79-88 (1982). These publications describe methods for crystallizing interferon alpha-2 from polyethylene glycol at low temperature or from a phosphate buffer solution by adjusting the pH or temperature. These methods normally provide needle crystals which cannot be well characterized by X-ray diffraction techniques. The Miller et al. article also mentions crystalline interferon alpha-2 in a "prismatic form".
In general, the methods for crystallizing proteins such as interferon have been found to be unpredictable. For example, Kung et al., U.S. Pat. No. 4,672,108 (1987) specifically states in column 1, lines 52-64:
"Numerous techniques have been developed for the crystallization of proteins, however, no generalized procedure has been discovered, and many proteins remain uncrystallized. Thus, crystallization of proteins is an unpredictable art utilizing tdal and error procedures among many possible alternative methodologies. PA0 One of the most widely used approaches involves the addition to the protein solution of a crystallizing agent, which is commonly a salt, such as ammonium sulfate or ammonium citrate or an organic solvent, such as ethanol or 2-ethyl-2,4-pentanediol. However, such procedures do not provide a suitable means for producing crystalline human leukocyte interferons." (Emphasis added.)